1. Field of the Invention
The present invention relates to a paint priming method utilizing a gas soft nitriding to form on the surface of the body to be treated a nitride layer of mainly .epsilon. phase, preferably with no carbon adhering, and adapted for direct painting thereon.
2. Description of Relevant Art
In general, washing for the primary purpose of de-waxing machine oils on the surface of a body to be treated, and a presubstrate treatment prior to painting to prevent rusting after painting and to improve the paint film strength, are effected in a known painting process.
The conventional method of presubstrate treatment for painting is very inconvenient because it requires many steps, such as a de-waxing step, a chemical film-formation step, and a finishing step, etc. The equipment required to perform such steps is quite substantial. In addition, many chemical agents, such as a de-waxing reagent, an acid washing reagent, and reagents for chemical film-forming, etc., are required. Thus, the cost becomes high due to the substantial number of devices and chemical agents required. Further, a great deal of equipment is required for waste water treatment due to the use of reagents. All of the foregoing factors result in a very high total cost of the painting.
A gas soft nitriding method has been known as a surface treating technique for a ferrous member so as to build up the fatigue resistance and the wear resistance thereof. This method is able to provide the ferrous member with a soft nitriding treatment which is relatively safe by means of introducing a carrier gas containing nitrogen. Such gas soft nitriding forms a layer of crystal structure known as ".epsilon. phase" composed of iron, nitrogen and carbon (Fe-N-C) in the surface part of the ferrous member to be treated.
The present invention effectively solves the problems attendant conventional presubstrate treatment by applying the gas soft nitriding treatment for the presubstrate treatment for painting.
In accordance with the principles of the present invention, when a ferrous member is treated by gas soft nitriding, the surface roughness is increased, the surface is cleaned with the oils on the surface being combusted, the nitride layer formed in the surface part has a high rust resistance, and such treatment is able to be effected by completely dry means. Further, the nitride layer thus formed may be used as a painting presubstrate on which direct painting can be effected, if the nitride layer is of a desired phase and a desired thickness, with no carbon adhesion to the surface being present.
The invention is therefore applicable to a body which requires a gas soft nitriding treatment, but may also be applied to a body which only preferably has such treatment. The gas soft nitriding treatment and the painting treatment can be effected through continuous steps, and can also be applied in combination with a previous process of gas nitriding, such as a brazing step.